Magnet wire enamels constitute unique insulating materials. These enamels are applied to wire substrates as an uncured resin layer which is then cured by exposing it to elevated temperatures. These insulating layers have typical thicknesses ranging in tenths of a mil and the entire insulating layer may comprise a plurality of individually applied and cured layers of the same or different resins. These insulating layers must possess certain qualities such as high dielectric strength, excellent flexibility, thermal stability and must be capable of uniform application to the wire substrate.
In the past, enamels have been prepared from polyamide resin materials or modified polyamide resins, however, many of these enamels had to be prepared as low solids materials to maintain the desired viscosity which would allow easy and uniform application to the wire. Since presently it is desirable to increase the solids content of these enamels, thereby lowering the amount of solvents used and thereby the cost of the enamel, such polyamide resins have been equilibrated in a reaction between the polyamide and a monoethanolamine as shown in reaction 1: ##STR1## This reaction requires that an excess of monoethanolamine be present during the reaction. The excess must be neutralized prior to the coating being applied to the wire and is accomplished by adding acetic anhydride to the solution which reacts with the monoethanolamine in the primary reaction as shown in reaction 2 below: ##STR2## However certain additional side reactions also occur during this neutralization process, the most important of which is shown below in reaction 3. ##STR3## This reaction being reversible at relatively low temperatures (about 80.degree. C. to about 100.degree. C.) means that during the oven curing of the coating (which temperatures exceed those to reverse the reaction) acetic acid and monoethanolamine are produced. The acetic acid is easily volatilized out of the coating during curing but the ethanolamine is not completely removed and remains behind. This material is known to be deleterious to many wire varnish curing mechanisms. Therefore, the use of this nylon as a wire coating (particularly for fine wires) results in wire having undercured coatings which do not effectively adhere to the wire or to wire whose mechanical and electrical properties are prematurely reduced during use.
In addition, the resulting wire coatings do not exhibit uniform properties from batch to batch and quality control is difficult. One evidence of this is the fluctuation in the mandrel pull test used to test the ability of wire coating to withstand stress cycles without exhibiting cracks or defects in the wire coating. Coatings formed using the conventional monoethanolamine approach to preparing the equilibrated polyamide exhibits wide fluctuations in their ability to pass such tests and in many instances the wire is unacceptable.
Therefore, what is needed in the art is a method by which the polyamide resin may be equilibrated without the introduction or resulting by-products which will affect the quality of the final coated wire product.